1. Field of the Invention
The present invention relates generally to a disc record reproducing apparatus, and is directed more particularly to a reproducing apparatus for a disc record on which, for example, a PCM (pulse code modulation) signal such as a PCM audio signal in the form of an RLLC (run length limited code) is recorded.
2. Description of the Prior Art
In the art, when a PCM signal is recorded on a disc, there are two methods one of which is a recording such that the angular velocity of the disc is selected to be constant and the other of which is a recording such that the line velocity of the disc is selected to be constant. In the case of the constant line velocity recording, its recording density is greatly improved as compared with the constant angular velocity recording, but in the case of the constant line velocity recording, it is necessary to reproduce the disc record with its line velocity being constant, which results in the rotation control of the disc record during reproduction is difficult.
In the case of a video disc, the rotation control thereof is carried out in a manner such that a reproduced signal is demodulated to provide a composite video signal, the synchronizing signal is then separated from the composite video signal, and the frequency of the synchronizing signal is made constant.
In the case of a PCM audio signal whose base band is recorded, it is generally a binary signal, so that the above method can not be employed. If the PCM audio signal is not recorded as the base band recording but is recorded after it is converted into a quase or pesudo video signal containing a synchronizing signal, the method which is the same as that in the case of the video disc can be employed. However, in this case the recording density is lowered, which will cancel the advantage of the constant line velocity recording.
On the other, a method may be considered during reproducing where the radial position of a pick-up device on a disc record is detected by using a mechanical position detecting means and, the rotation of the disc record is controlled by the detected output, a clock component is extracted from a reproduced signal, and the extracted component is used as a comparing signal to further control the rotation of the disc record. However, the employment of such mechanical position detecting means greatly increases the cost of the apparatus.